Electrically-conductive ink formulations that can be thermally converted into metallic patterns forming electrical interconnects, components or wireless sensors have been previously described in patent literature and are commercially available. However, these formulations are often composed of metal particles (flakes/nanopowders), organic metal precursors, binders, additives and solvents. The presence of resinous binders and other additives in the conductive formulations increase the resistance and interferes with the RF/(Radio Frequency)/AC (alternating current) characteristics of the printed elements and typically require curing at high temperature in order to achieve sufficient conductivities. Therefore, formulations that include resinous binders and other additives often are not suitable for temperature-sensitive substrates like plastics or the RF (radio frequency)/AC (alternating current) applications. The utility of these commercial printable conductive formulations have been limited mainly to direct current applications (printed circuitry and interconnections for electronic devices). Further, conventional low-temperature electrically conductive adhesives (ECAs) have limitations such as relatively low electrical conductivity and unstable contact resistance.
The use of inductively-coupled passive radio frequency sensors previously has been described in the area of structural health monitory (crack detection). For example, U.S. Pat. No. 6,973,838, the entire contents of which are incorporated herein by reference, discloses a non-contact crack sensor to detect cracks in composite materials. However, the practical and cost-effective fabrication of these sensors has not been described in any patent or relevant sensor literature. Sensors for wireless structural health monitoring typically are fabricated by photolithography and laser micromachining techniques that are cumbersome or expensive.
There is a need for electrically conductive formulations that overcome or minimize the above-referenced problems.